Batteries have become increasingly popular over the last decade due to the advent of a myriad of portable electronic devices. Especially lithium ion secondary batteries have become the predominant power source for the devices such as cellular phones, notebook or laptop computers, camcorders, digital cameras and more.
Lithium ion secondary batteries contain a spirally wound electrode assembly installed in a suitable battery case that has to be filled with non-aqueous electrolyte. In the battery case with the electrode assembly installed in it, a large number of small voids are formed. Typically, the air in the internal space of the battery is being drawn out by means of applying a vacuum and the electrolyte is filled under vacuum against atmospheric pressure. However, long time is required for completely exhausting the voids. Further, some time is required until the non-aqueous electrolyte has permeated into the electrode assembly, and it is very difficult to inject the electrolyte within a short time.
JP-07099050(A) describes an apparatus, comprising a battery arranged in a chamber and with an electrolyte to be injected into it, and a predetermined amount of electrolyte is filled in an electrolyte reservoir mounted on the injection nozzle. Then, the pressure in the chamber is reduced, and gas such as the air in the electrolyte or the electrode assembly is removed. Then, the pressure is restored to atmospheric pressure, and the electrolyte is injected. In this apparatus, however, a funnel-like member with a reservoir corresponding to the amount of the electrolyte to be injected is mounted while the top portion of the battery base with the battery element is opened, and the space inside the battery case is exhausted. A part of the electrolyte is injected into the battery case before exhausting and is permeated into the battery element. As a result, the exhausting from the voids in the electrode assembly is insufficient because of the presence of the electrolyte. Exhausting is performed while the electrolyte is present in the reservoir, which comprises a funnel-like member on the top portion of the battery case, and the pressure is applied as the atmospheric pressure. As a result, air bubbles are generated when the air passes through the funnel-like unit from inside the battery case, and these air bubbles are sent into the battery.
U.S. Pat. No. 5,738,690 teaches a method of filling a battery cell for electric vehicle applications. In this method, a special apparatus arrangement enables vacuum assisted filling of electrolyte against atmospheric pressure. The time required and the achieved electrolyte fill level are not mentioned, but will be limited by the driving force of atmospheric pressure.
U.S. Pat. No. 6,497,976 describes a method for electrolyte filling of a small size rectangular battery with a small electrolyte injection hole in the battery case. In this method, the battery case is exhausted by vacuum followed by electrolyte injection under pressure of up to 2 kgf/cm2 (196 kPa) to enable quick filling of electrolyte. Filling according to this method can be achieved in 60 seconds, but cannot achieve maximum electrolyte fill levels. A problem in this filling method lies in that the high pressure is maintained till the end of the filling process, wherein the small voids cannot be filled as the air cannot escape.
One result of the presence of voids after filling is the wide deviation range of battery weight which is due to the fact that different individual battery cells can take different amount of electrolyte. The battery performance is negatively influenced by the presence of voids and by the incomplete filling of the available battery space with electrolyte.
Since maximum electrolyte fill level is very important to battery performance, some companies have started to go through 2 or 3 electrolyte fill operations on their existing vacuum fillers, which involves considerable slow down in production speeds and increased parts handing.